In conventional engine starters, it has been customary to arrange the output shaft, which carries an axially slidable pinion adapted to mesh with a ring gear, and the solenoid device for axially driving the pinion, in a mutually parallel relationship. However, because such bi-axial engine starters have a solenoid device which extends radially from the electric motor, and therefore inevitably have a substantial radial dimension, there have been severe restrictions in ensuring a sufficient space for mounting the engine starter.
To overcome such a problem, it has been proposed to provide a coaxial starter having an annular solenoid device surrounding the output shaft (see for example Japanese Patent Application Laid-Open Publication No. 8-319926 filed by the same applicant).
In such a coaxial type engine starter, the solenoid device is disposed between the pinion and the DC electric motor so that when activated, it moves the pinion axially into mesh with the ring gear of the engine. The pinion is always urged by a return spring or the like in a direction away from the ring gear of the engine so that when the solenoid device is deactivated the pinion is disengaged from the ring gear. In order to define the rest position of the pinion (or the position when the pinion is not moved by the activated solenoid device) against the force of the return spring, an annular stopper usually made of resin material is mounted at a position between the solenoid device and the pinion. Preferably, this stopper is secured not only in the axial direction but also in the rotational direction and connected to the solenoid device so that the stopper also serves to secure the solenoid device in the rotational direction.
The engine starter comprises a gear cover for accommodating the pinion, solenoid device and other component parts. This gear cover typically has a substantially cylindrical inner surface, and comprises a smaller diameter part for accommodating the pinion and a larger diameter part for accommodating the solenoid device, with an annular shoulder surface formed at the boundary between the smaller and larger diameter parts. In mounting the solenoid device in the gear cover, the solenoid device is forced into the larger diameter part of the gear cover until its end surface abuts the shoulder surface defined in the gear cover. In this way, the shoulder surface in the gear cover determines the axial position of the solenoid device.
In order to favorably dispose the stopper between the solenoid device and the pinion, the stopper is formed with a plurality of tongues projecting radially outwardly from its outer periphery, and the shoulder surface in the gear cover is formed with corresponding recesses for accommodating the tongues, so that each of the tongues of the stopper can be fixedly held between the corresponding recessed part of the shoulder surface and the end surface of the solenoid device.
However, if the tongues are too thin, the stopper cannot be steadily mounted. On the other hand, if the tongues are too thick, although the stopper can be steadily mounted, such tongues may prevent the solenoid device from abutting the shoulder surface in the gear cover. In such a case, the axial position of the solenoid device is varied depending on the thickness of the tongues, resulting in variation in the armature's stroke which in turn may create problems such as insufficient thrust force applied to the pinion or failure to achieve a required motion of a moveable contact plate of the switch unit which is operated in connection with the armature so as to selectively supply electric power to the motor. Although the stopper is typically made of resin material, deformation of the tongues of the stopper has been limited and insufficient to absorb deviation in size of the tongues from the standard one, because in order to secure the stopper in the rotational direction, the tongues have been designed so as to fit in their corresponding recesses, leaving little space therebetween to accommodate enough deformation of the tongues.